A good starting point could be to focus on the relationship between Luna and Ginny first. Show how they are different yet complementary in their magical abilities and personalities. Then, introduce the Muggle male as an outsider who disrupts their normal routine. This disruption can be the catalyst for the story. For instance, he might have found a lost magical item and they need to get it back before it causes chaos in the Muggle world.
Begin with a conflict. Maybe the Muggle male has unknowingly gotten himself into trouble with a dark wizard. Luna and Ginny hear about it through the grapevine in the wizarding world. This immediately sets up a situation where they need to rescue him. You can then describe their first encounter, how Ginny might be more cautious while Luna is more curious about this Muggle in trouble.